This invention relates to the field of lithographic printing and more particularly to an improved method for increasing the abrasion and wear-resistance of lithographic plates by providing a tough, durable, solvent resistant layer at the outside of a lithographic image.
Images obtained from the exposure and development of conventional lithographic light-sensitive materials are often relatively fragile and consequently abrade and deteriorate relatively rapidly under printing conditions. Diazo resin sensitizers, for example, such as those produced by condensation of formaldehyde and diazodiphenylamine, provide sharp images and enjoy substantial commercial success, but the images are quite fragile and not well adapted for long run printing operations.
In order to impart improved image toughness for diazo resin sensitized plates, "additive" or lacquer developers are commonly used. Such developers contain a resin component which coats out over the image upon development of the plate to provide an overcoating having a significantly greater toughness than that exhibited by the light-exposed diazo resin. A variety of organic resins including polyvinyl chloride have been utilized in lacquer developers. Currently, however, epoxy resins, particularly those derived from epichlorohydrin and bisphenol A are most commonly used as the resin component of a lacquer developer. Certain of the commercially available lacquer developers are emulsions which contain both organic solvents as a vehicle for the epoxy resins and acids for desensitization of the underlying substrate in the areas not exposed to light.
Epoxy resins have also been utilized as overcoatings over diazo resin sensitizing coatings in so-called "subtractive" lithographic plates. Such a plate is described, for example, in Larson U.S. Pat. No. 3,136,637. These plates are referred to as "subtractive" because the resin overcoating which imparts improved durability to the plate is present on the plate before exposure, and development involves mere removal of light-sensitive and overcoating material in the unexposed portions of the plate without any addition to the image of components deposited from the developer.
Although epoxy and other resin overcoatings are widely utilized and generally effective to impart improved durability to lithographic images, the strength and effectiveness of epoxy coatings is limited by the relatively low molecular weight thereof. In order to permit the application of epoxy resin in an additive lacquer developer or to permit removal of the epoxy upon development of a subtractive plate such as that described in the aforesaid Larson patent, it is necessary that the resin be of relatively low molecular weight. As a result, the durability of the epoxy film at the image surface is limited by the physical cohesive forces of the resin molecules as the epoxy congeals into a film upon evaporation of the solvent vehicle from which it is applied. Such films do not exhibit the toughness, durability and abrasion resistance of a cross-linked epoxy resin material. Moreover, such low molecular weight resins are subject to attack by strong solvents such as cyclohexanone, ethylene glycol alkyl ethers, acrylic monomers and toluene. Thus, images provided with epoxy resin surface coatings are subject to deterioration on exposure to strong solvents contained in blanket and roller washes and in certain inks, for example, the U.V. curable inks that contain acrylic monomers.